Illumination device for projection system and method for fabricating

ABSTRACT

An illumination system for a microlithographic stepper has a light source that emits light of selected wavelength(s) along an optical path toward a photomask. An aperture mask is positioned in the path of the illumination light and between the light source and the photomask. The aperture mask has a dithered pattern of pixels. The intensity of the pattern controls the illumination of the photomask. The masking aperture pattern defines one or more zones of illumination. Each zone has elements that are patterned in accordance with a selected wavelength of incident light to diffract the incident light into an illumination pattern for illuminating a photomask. Each of the elements is constructed with a matrix of pixels. In the preferred embodiment the array of pixels is 8×8. The number of elements is generally greater than 3×3.

CROSS-REFERENCE TO RELATED APPLICATIONS

This invention claims the benefit of the priority date of the followingU.S. Provisional Application No. 60/105,281 filed Oct. 22, 1998 and No.60/119,780, filed Feb. 11, 1999.

BACKGROUND OF THE INVENTION

Optical lithography has been one of the principal driving forces behindthe continual improvements in the size and performance of the integratedcircuit (IC) since its inception. Feature resolution down to 0.30 μm isnow routine using the 365 nm mercury (Hg) i-line wavelength and opticalprojection tools operating at numerical apertures above 0.55 withaberration levels below 0.05 λRMS OPD. The industry is at a point whereresolution is limited for current optical lithographic technologies. Inorder to extend capabilities toward sub-0.25 μm, modifications in sourcewavelength, optics, illumination, masking, and process technology arerequired and are getting very much attention.

However, as devices get smaller, the photomask pattern becomes finer.Fine patterns diffract light and thus detract from imaging the photomaskonto the surface of a wafer. FIG. 1 a shows what happens when aphotomask with a fine pattern 6 having a high frequency (pitch 2d isabout several microns), is illuminated through a projection lens system7. The fine pattern 6 is illuminated along a direction perpendicular tothe surface thereof and it diffracts the light that passes through themask 6. Diffraction rays 3-5 caused by the pattern include a zero-thorder diffraction ray 5 directed in the same direction as the directionof advancement of the input ray, and higher order diffraction rays suchas positive and negative first order diffraction rays 3, 4, for example,directed in directions different from the input ray. Among thesediffraction rays, those of particular diffraction orders such as, forexample, the zero-th order diffraction ray and positive and negativefirst order diffraction rays 3, 5, are incident on a pupil 1 of theprojection lens system 7. Then, after passing through the pupil 1, theserays are directed to an image plane of the projection lens system,whereby an image of the fine pattern 6 is formed on the image plane. Inthis type of image formation, the ray components which are contributableto the contrast of the image are higher order diffraction rays. If thefrequency of a fine pattern increases, it raises a problem that anoptical system does not receive higher order diffraction rays.Therefore, the contrast of the image degrades and, ultimately, theimaging itself becomes unattainable.

As will be shown below, some solutions to this problem rely upon shapingthe rays of light impinging the photomask in order to provide off-axisillumination to compensate for the lost contrast due to diffraction.These techniques rely upon optical systems for shaping the rays thatilluminate the photomask.

In considering potential strategies for sub-0.25 μm lithography, theidentification of purely optical issues is difficult. Historically, theRayleigh criteria for resolution (R) and depth of focus (DOF) has beenutilized to evaluate the performance of a given technology:R=k₁λ/NADOF=+/−k₂λ/NA²where k₁ and k₂ are process dependent factors, λ is wavelength, and NAis numerical aperture. As wavelength is decreased and numerical apertureis increased, resolution capability improves. Considered along with thewavelength-linear and NA-quadratic loss in focal depth, reasonableestimates can be made for system performance. Innovations in lithographysystems, materials and processes that are capable of producingimprovements in resolution, focal depth, field size, and processperformance are those that are considered most practical.

The Hg lamp is a source well suited for photolithography and is reliedon almost entirely for production of radiation in the 350-450 nm range.Excimer lasers using argon fluoride (ArF) and krypton fluoride (KrF),which produce radiation at 193 nm and 248 nm, respectively, are alsoused. DUV lithography at 248 nm is now being implemented intomanufacturing operations and may be capable of resolution to 0.18 μm.

The control of the relative size of the illumination system numericalaperture has historically been used to optimize the performance of alithographic projection tool. Control of this NA with respect to theprojection systems objective lens NA allows for modification of spatialcoherence at the mask plane, commonly referred to partial coherence.This is accomplished through specification of the condenser lens pupilsize with respect to the projection lens pupil in a K ōhler illuminationsystem. Essentially this allows for manipulation of the opticalprocessing of diffraction information. Optimization of the partialcoherence of a projection imaging system is conventionally accomplishedusing full circular illuminator apertures. By controlling thedistribution of diffraction information in the objective lens with theilluminator pupil size, maximum image modulation can be obtained.Illumination systems can be further refined by considering variations tofall circular illumination apertures. A system where illumination isobliquely incident on the mask at an angle so that the zero-th and firstdiffraction orders are distributed on alternative sides of the opticalaxis may allow for improvements. Such an approach is generally referredto as off-axis illumination. The resulting two diffraction orders can besufficient for imaging. The minimum pitch resolution possible for thisoblique condition of partially coherent illumination is 0.5 λ/NA, onehalf that possible for conventional illumination. This is accomplishedby limiting illumination to two narrow beams, distributed at selectedangles. The illumination angle is chosen uniquely for a given wavelength(λ), numerical aperture (NA), and feature pitch (d) and can becalculated for dense features as sin⁻¹ (0.5 λ/d) for NA=0.5 λ/d. Themost significant impact of off axis illumination is realized whenconsidering focal depth. In this case, the zero-th and 1st diffractionorders travel an identical path length regardless of the defocus amount.The consequence is a depth of focus that is effectively infinite.

In practice, limiting illumination to allow for one narrow beam or pairof beams leads to zero intensity. Also, imaging is limited to featuresoriented along one direction in an x-y plane. To overcome this, anannular or ring aperture has been employed that delivers illumination atangles needed with a finite ring width to allow for some finiteintensity. The resulting focal depth is less than that for the idealcase, but improvement over a full circular aperture can be achieved. Formost integrated circuit application, features are limited to horizontaland vertical orientation, and a four-zone configuration may be moresuitable. Here, zones are at diagonal positions oriented 45 degrees tohorizontal and vertical mask features. Each beam is off-axis to all maskfeatures, and minimal image degradation exists. Either the annular orthe four-zone off-axis system can be optimized for a specific featuresize, which would provide non-optimal illumination for all others. Forfeatures other than those that are targeted and optimized for, higherfrequency components do not overlap, and additional spatial frequencyartifacts are introduced. This can lead to a possible degradation ofimaging performance.

When considering dense features (1:1 to 1:3 line to space duty ratio),modulation and focal depth improvement can be realized through properchoice of illumination configuration and angle. For true isolatedfeatures, however, discrete diffraction orders would not exist; insteada continuous diffraction pattern is produced. Convolving such afrequency representation with either illumination zones or annular ringswould result in diffraction information distributed over a range ofangles. Truly isolated line performance is, therefore, not improved withoff-axis illumination. When features are not completely isolated buthave low density (>1:3 line to space duty ratio), the condition foroptimum illumination will not be optimal for more dense features.

Furthermore, the use of off-axis illumination is generally not requiredfor the large pitch values that correspond to low density geometry. Asdense and mostly isolated features are considered together in a field,it follows that the impact of off-axis illumination on these featureswill differ, and a large disparity in dense to isolated featureperformance can result.

One approach to generate off-axis illumination is to incorporate a metalaperture plate filter into the fly eye lens assembly of the projectionsystem illuminator providing oblique illumination. A pattern on such ametal plate would have four quadruple openings (zones) with sizing andspacing set to allow diffraction order overlap for specific geometrysizing and duty ratio on the photomask, as disclosed in JP patentLaid-Open (KOKAI) Publication No. 4-267515. Such an approach results ina significant loss in intensity available to the mask, loweringthroughput and making the approach less than desirable. Additionally,the four circular openings need to be designed specifically for certainmask geometry and pitch and would not improve the performance of othergeometry sizes and spacings. Large levels of mask biasing or maskoptical proximity correction (OPC), where mask features arepre-distorted to produce desired image characteristics, would berequired to allow for use of this approach with a variety of features.Filtering, by limiting its effective area, reduces the effect of the flyeye diffuser on maximizing illumination uniformity. Illuminationuniformity may be degraded. This approach also limits the illuminationprofile to one having holes in a metal plate. That is, the masking metalmust remain contiguous. The previous work in this area describes suchmethods using either two or four openings in the aperture plate:EP0500393, U.S. Pat. Nos. 5,305,054, 5,673,103, 5,638,211, EP0496891,EP0486316, U.S. Pat. No. 379,252.

Another approach to off-axis illumination using the four-zoneconfiguration, which is disclosed in U.S. Pat. No. 5,627,625, is todivide the illumination field of the projection system into beams thatcan be shaped to distribute off-axis illumination to the photomask. Byincorporating the ability to shape off-axis illumination, throughput andflexibility of the exposure source is maintained. Additionally, thisapproach allows for illumination that combines off-axis and on-axis(conventional) characteristics. By doing so, the improvement to densefeatures that are targeted with off-axis illumination is lesssignificant than straight off-axis illumination. The performance of lessdense features, however, is more optimal because of the more preferredon-axis illumination for these features. The result is a reduction inthe optical proximity effect between dense and isolated features.Optimization is less dependent on feature geometry and more universalillumination conditions can be selected.

A problem with this divided illumination approach is that it requiresreconfiguration of the illumination system of a projection tool, a taskthat is not practical on existing tools or systems designed with otherillumination systems. Additionally, the use of divided beam illuminationlimits the fine control of beam shape, size, and position to that whichis possible with optical components utilized in the system. Variationsin shape, size, position, number of beams, maximum aperture size, orother feature or lens specific variations to the illumination intensityprofile become difficult without significant mechanical modifications.Some variations may not be practical or possible with this approach.This has significantly limited the acceptance or use of this approach inmost integrated circuit fabrication operations.

SUMMARY OF THE INVENTION

A shaped illumination approach is described that allows for off-axisillumination of a photomask in a projection exposure tool. It isnecessary to control both the off-axis and the on-axis character of theillumination for instance so that dense line performance can be improvedand more isolated line performance can be maintained, i.e., opticalproximity effect (hereinafter “OPE”) is kept to a minimum. Minimal OPEcorrection is desired to reduce mask complexity. There is also a desirefor a flexible technique that can be incorporated into most existing orfuture generation projection exposure tools with a minimum amount ofillumination system retrofitting. It is important that such animprovement be easily removed to allow a return to original operationconditions since it is expected that a given projection exposure systemwould be used for a variety of applications. Such an approach can leadto optimizing illumination conditions, which can be incorporated into anexposure system as a more permanent condition. The invention alsoprovides an improvement that allows for fine adjustment or modificationto accommodate mask-specific or lens-specific characteristics isimportant as resolution and focal depth requirement are pushed beyondthe capability of conventional optical lithography. Maintainingillumination throughput is also critical, as any loss in intensity willresult in increased exposure time requirements.

The invention provides such a solution. It modifies existingillumination system by adding a masking aperture in the illuminationpupil plane, fabricated as an optical component reticle, patterned anddithered into a large number of elements to allow for control of theprojected light distribution at the mask plane and inserted at thecondenser lens pupil plane. This masking aperture comprises atranslucent substrate and a masking film. The distribution of theintensity through the masking aperture in the illumination pupil plane,is determined to provide optimized illumination. The illumination regionor regions exhibit varying intensity, which is accomplished by creatinga half-tone pattern via pixelation of the masking film, thereby allowingfor maximum variation in illumination beyond the simple binary (clear oropaque) possible with earlier pupil plane filtering approaches.

More specifically, the invention includes an aperture mask for anillumination system to provide controlled on-axis and off-axisillumination. The aperture mask acts as a diffraction element. Theaperture mask is divided into an array of elements and each elementcontains an array of pixels. Each of the elements is constructed with amatrix of pixels. In the preferred embodiment the array of pixels is8×8. The number of elements in the illumination array are generallylarger than 3×3 and array sizes of 21×21 and 51×51 are an example, whichcorrespond to 441 and 2601 elements respectively. The elements arepatterned in accordance with a selected wavelength of incident light todiffract the incident light into an illumination pattern forilluminating a photomask.

The intensity is modulated by the intensity state of pixels within eachelement. The highest intensity element has all pixels clear or maximumintensity. Light of suitable wavelength passes through withoutattenuation. An element with 64 pixels having dark or minimum intensityattenuates or blocks all light. Elements of intensity between none (0%)and all (100%) are created by the state of the pixels in a givenelement. Random patterns and other patterns between elements may produceartifacts similar to moire patterns. Such artifacts are undesired. Idiscovered that a dithered pattern using position dependent thresholdsproduced illumination patterns that had little or no artifacts.

I also discovered that traditional, optical shaping systems such asbeam-splitters or diffractive optical elements and my diffractionshaping system can each be improved by adding an illumination aperture.A square illumination aperture shows maximum improvement. It is locatedat or near the pupil of the illumination system. An aperture with alarge, central square opening and an opaque border is inserted at thecondenser lens pupil proximate to the fly's eye lens. It can also bedesigned into the masking aperture. The resulting illumination patternfills the corners and squares the edges of the illumination pupil andlimits the non-optimal frequency spreading character along the x and yaxes while optimizing the off-axis illumination angles. The squareillumination aperture is especially helpful for imaging features thatare oriented along x and y directions in the mask plane. The use of acentral obscuration (square and also round shaped) applied in the samelocation can similarly be achieved and can lead to performanceimprovements described hereinafter. Furthermore, any combination ofoff-axis illumination with a square pupil or obscuration has potentialto improve performance for geometry oriented in the x-y direction. Thiscan include, but is not limited to, round zones, elliptical zones,square zones, and annular slots (that is an annular ring masked off on xand y axis to form arc-shaped zones).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a 1 is a schematic view of an illumination system for a photomask.

FIG. 1b shows the zero-th and first positive and first negativediffraction rays.

FIG. 1c is a schematic view of a four-zone illumination arrangement.

FIG. 2 is a drawing showing diffraction orders (first and zero) for 130mn x-oriented features using four-zone illumination, plotted in thefrequency plane of the objective lens. Center sigma is 0.68 and radiussigma is 0.13, chosen to accommodate feature pitch values from 260 to455 nm.

FIG. 3 is a drawing of the continuous tone intensity distribution for amasking aperture based on four distributed-intensity zones.

FIG. 4 is a plot of the x-y distribution of dithered bilevel maskingcells for an illumination aperture consisting of four circular normaldistributed-intensity zones placed at diagonal positions correspondingto off-axis illumination for geometry oriented in horizontal andvertical directions. The axes are divided into relative distances usingthe center and the edges of the mask.

FIG. 5 is a three dimensional plot of the x-y distribution shown in FIG.4.

FIG. 6 is a drawing of surface contours of the continuous tone intensitydistribution for a masking aperture based on two distributed-intensityzones where the maximum off-axis angle is limited to one direction.

FIG. 7 is a plot of the x-y distribution of dithered, bilevel maskingcells for an illumination aperture consisting of four elliptical normaldistributed-intensity zones placed at diagonal positions correspondingto off-axis illumination for geometry oriented in horizontal andvertical directions. The axes are divided into relative distances usingthe center and the edges of the mask.

FIG. 8 is a contour plot of the x-y distribution shown in FIG. 7.

FIG. 9 is a three dimensional plot of the x-y distribution for anillumination aperture consisting of four stepped squaredistributed-intensity zones placed at diagonal positions correspondingto off-axis illumination for geometry oriented in horizontal andvertical directions.

FIG. 10 is a contour plot of the x-y distribution for an illuminationaperture consisting of four stepped square distributed-intensity zones,as described for FIG. 9.

FIG. 11 is a plot of the x-y distribution of dithered, bilevel maskingcells for an illumination aperture consisting of four elliptical normaldistributed-intensity zones, as described for FIG. 9.

FIG. 12 is a schematic representation of the bilevel representation of65 gray levels using the ordered dithering algorithm.

FIG. 13 is a graph showing normalized aerial image log-slope (NILS) vsfocus for 130 mn features using four-zone illumination, σ_(c)=0.68 andσ_(r)=0.20.

FIG. 14 is a graph showing normalized serial image log-slope (NILS) vs.focus for 130 nm features using distributed-intensity four-zoneillumination (σ_(c)=0.68 and σ_(r)=0.30) and a circular hard stop(σ=0.8).

FIG. 15a is a graph showing normalized image log slope (NILS) vs. focusfor 130 nm features using the illumination shown in FIG. 19.

FIG. 15b is a graph showing normalized image log slope (NILS) vs. focusfor 130 nm features using the illumination shown in FIG. 23.

FIG. 15c is a graph showing normalized image log slope (NILS) vs. focusfor annular ring illumination of 150 nm features using 248 nm wavelengthand 0.63 NA, with duty ration of 1:1.5 to 1:5.

FIG. 15d is a graph showing normalized image log slope (NILS) vs. focusfor square ring illumination of 150 nm features using 248 nm wavelengthand 0.63 NA, with duty ration of 1:1.5 to 1:5.

FIG. 16 is a plot of the normalized image log slope, showing influenceof a circular four-zone (σ_(c)=0.7 and σ_(r)=0.2 on a zero intensityfield) with 0.1 waves of primary aberrations: astigmatism, tilt,spherical, and coma, through focus for a 130 nm features imaged with a193 nm imaging system using a numerical aperture of 0.6.

FIG. 17 is a plot of the normalized image log slope, using the bi-levelrepresentation of the distributed-intensity illumination described forthis invention.

FIG. 18 is a contour plot of circular gaussian zones used to optimizeenergy distribution in the pupil, where energy is concentrated in thecenter of the pupil and at off-axis angles.

FIG. 19 is a three dimensional plot of circular gaussian zones and asquare limiting zone used to limit the on-axis illumination componentwhile retaining off-axis zone energy.

FIG. 20 is a perspective view of a beamsplitter illumination system witha square aperture mask inserted in its pupil.

FIG. 21 shows how a square central obscuration can be included in anaperture to reduce the on-axis character of illumination and improve theperformance of dense features.

FIG. 22 shows the square ring, off-axis source which delivers optimaloff-axis illumination for dense features while providing on-axisillumination for more isolated features.

FIG. 23 shows how the square ring approach can be combined with thegaussian four-zone approach to emphasize comer zone effects.

FIG. 24 is a contour plot of a combined annular and gaussian four-zoneaperture to improve the off-axis character of annular illumination.

FIG. 25 is an illumination system using diffractive optical elements anda square aperture.

FIG. 26 is a 51×51 element array of a dithered half tone pattern.

DETAILED DESCRIPTION OF THE INVENTION

For given ranges of feature types and/or sizes (types being lines,spaces, contacts, and dense or isolated combinations of these) exposureconditions are optimized by determining the average off-axis angle toaccommodate all features. The distribution of off-axis angles is thendetermined based on the range of feature sizes of interest. As the rangeof feature sizes increases, the condition of off-axis illuminationapproaches a limit equivalent to the on-axis condition. Most commonly,duty ratios for a given feature size may range from 1:1 to 1:6line:space ratio. The spread of off-axis illumination angles isaccomplished by shaping zones (for the two or more zone, includingfour-zone) or rings (for the annular) to produce continuous intensitydistributions.

As an example of the design considerations, 130 nm features areconsidered using a 193 nm exposure wavelength and a 0.60 objective lensnumerical aperture. Duty ratios from 1:1 to 1:6 are included. Featuresare considered dense with a duty ratio less than 1:3. Table 1 shows thepitch values (p) for these dense features, along with the required axiscenter sigma and four-zone center sigma (σ_(c)) values for optimumoff-axis illumination. Center sigma values on axis are determined asλ/(2p·NA), as shown in FIG. 1. Since diffraction order placement isdetermined by the projection of the four-zone onto the x or y axis, thecenter four-zone sigma values are larger by a (2)^(1/2) factor.

In order to design an off-axis illumination configuration that canaccommodate and enhance the range of pitch values in Table 1, zoneposition and radius values must be chosen so that some amount of orderoverlap occurs for each case. This can be accomplished if the four-zonecenter σ_(c) and the radius σ_(r) values are set as follows:σ_(c)=SQRT(2)*(0.61+0.35)/2=0.68σ_(r)>(0.61−0.35)/2=0.13

These choices for zone center and radius values correspond to thesituation where zero and first diffraction orders begin to overlap forthe extreme pitch values of 260 and 455 nm, as shown in FIG. 2. The(2)^(1/2) term does not factor into determination of σ_(r) values sincethe zone radius is projected directly onto the axes. The extent of orderoverlap for these extreme pitch values is, however, mostly ineffectual,and larger σ_(r) values are required to influence performance. A σ_(r)value of 0.20 allows for significant order overlap (˜20%) and is a morepractical starting value for further optimization.

When imaging of both dense and isolated features, illumination thatresembles both the strong four-zone and the conventional on-axisillumination is desirable. This can lead, for instance, to the four-zoneillumination design where circular zones are replaced with continuoustone zones, as shown in FIG. 3.

TABLE 1 Pitch and sigma values for 130 nm features and 1:1 to 1:2.5 dutyratios. Duty Pitch (nm) axis sigma four-zone sigma 1:1 260 0.61 0.871:1.5 325 0.50 0.71 1:2 390 0.42 0.59 1:2.5 455 0.35 0.49

The masking aperture of this invention is a bilevel representation ofthe desired intensity distribution in the illuminator. It is desired toresemble a near continuously varying transition from open to opaqueareas. To achieve this result, the illumination pattern is divided intoa large number of elements and each element is a matrix of pixels.Dithering or pixelation of the continuous distribution of intensity isused for translation to the binary or bilevel masking aperture. Theelement array is large, consisting of, for instance, 5×5, 7×7, 9×9,11×11, 21×21 or 51×51 elements, but not limited to these cases. Theillumination profile is divided into such an element array. Individualmasking pixels are small, on the order of 10 to 100 μm, and are eithertranslucent or opaque. Their size is dependent on the size of thephysical masking aperture. The continuous tone nature of theillumination intensity profile is translated by controlling the spatialdensity of the bilevel display states on the masking aperture. Severaldecision rules may be implemented to produce the output distribution onthe masking aperture. A fixed threshold technique is simplest in form,but an ordered dithering approach may be used to most effectivelytranslate a continuous tone intensity profile into a bilevel maskingaperture representation. Intensity values are compared to aposition-dependent set of threshold values, contained in a n×n dithermatrix. A set of selection rules repeats the dither matrix in acheckerboard arrangement over the illumination field. One key to thisapproach is the generation of a bilevel representation of the continuoustone image with the minimal amount of low spatial frequency noise. Inother words, the occurrence of texture, granularity, or other artifactsis reduced to a minimum, allowing for the critical control ofillumination uniformity demanded in projection exposure tools.

The resulting bilevel representation of the continuous tone off-axisand/or on-axis illumination profile is then suitable for recording intoa photo-sensitive or electron beam-sensitive resist material through useof mask pattern generator. Other approaches might use lithographictechniques common to lithographic or printing technologies. Such aresist material, when coated over an opaque film or translucentsubstrate, can allow for pattern delineation and creation of the maskingaperture.

In the present invention, the existing intensity distribution at thepupil plane of an illumination system for a projection exposure tool ismodified through use of a bilevel masking aperture containing a maskingcell representation of a continuous tone intensity distribution. FIGS. 4and 5 show such distributions where four distributed-intensity zonesallow for off-axis and on-axis illumination of a photomask that containsgeometry oriented in horizontal and vertical directions only. A maximumcircular dimension is defined by a limiting zone, designed to limit themaximum off-axis angle projected onto the mask. This is used to balancethe off-axis illumination provided to the mask with the degree ofcoherency of the on-axis illumination. Smaller geometry requires higherlevels of on-axis partial coherence, leading to larger limiting zones.The extent of the zone will generally correspond to positions near orbeyond the maximum zone or angular position for the off-axisillumination, generally in, but not limited to, the range from 0.5 to1.0. The intensity at any position located beyond this limiting zone isset to zero. The shape of this limiting zone is not necessarilycircular, and selection of the shape will depend on the extent offeature orientation at the mask. Features constrained to one orientationonly require limitation of off-axis illumination in one direction,resulting in assigning a value of zero to any element beyond therequired x or y value corresponding to the limiting angle, as shown inFIG. 6. This allows for maximum energy at angles with the desired rangeand can lead to improved imaging and throughput performance. Featuresconstrained to two orientations only require that the aperture limitoff-axis illumination angles in two orthogonal directions, leading to anon-circular or square two dimensional character of the limiting zone asshown in FIG. 19. The invention allows for this tailoring of theillumination.

If the existing illumination intensity distribution at the pupil planeof the illumination system is not uniform, the non-uniformity at theplane can be deconvolved in accordance with the invention to result in amasking aperture that also incorporates compensation for non-uniformity.For example, many steppers provide a pupil that is guaranteed uniform(+/−1%) for only 80% of its full opening. At 85% open, the uniformity ofillumination may vary up to +/− 10% or more. With the invention, thenon-uniformity may be canceled or reduced to an acceptable level.

The overlapping of the continuous intensity regions in the center of theillumination field produces the on-axis character required for lessdense features. The central intensity is generally greater than 0% andis commonly in the 10 to 50% range.

Illumination zones within the masking aperture control the illuminationto mask and are designed to produce optimal off-axis, on-axis, orcombined illumination. This invention allows for an infinite variety andnumber of such zones. Some are most desirable.

Zones may be circular, elliptical, 45 degree elliptical (that is,elliptical but oriented with axes at angles of 45 degrees and 135degrees), square, or other shapes dependent on the desired distributionof diffraction information to match mask geometry requirements orspecific lens behavior. The distribution of the energy in these zones orrings may be stepped, Gaussian, Lorentzian, or other similar shape. Thekurtosis of gaussian distributions may be normal (mesokurtic), narrow(leptokurtic), or flat-topped (platykurtic), or combinations of theseamong zones. Skewness, or departure from symmetry of the distributionmay be utilized for differential weighting of certain feature sizes.Circular symmetry may be best suited for most general cases andelliptical distributions can be utilized to accommodate x-ynonuniformities of the photomask or imparted by the projection lens (aresult for instance of astigmatic or comatic aberration). FIGS. 7 and 8show four elliptical normal distributed-intensity zones places atdiagonal positions corresponding to off-axis illumination for geometryoriented in horizontal and vertical directions. FIG. 18 shows howrotated elliptical gaussian zones can offer improvements in intensitydistribution for x and y oriented geometry by concentrating some energyin the center of the pupil and the remaining energy distributed atfour-zone angles. Here, rotation of the zone axis allows for asymmetrical distribution. In this case, energy on the x/y axes islimited, reducing the non-optimal, on-axis illumination of targeteddense features. An increases in the efficiency of zero and firstdiffraction order overlap can result from such an illuminationdistribution.

For imaging of geometry in two directions only (x and y only forinstance), there is only a need to spread diffraction order informationin the direction of geometry. By limiting zone intensity distribution tox and y directions, resulting in continuous intensity or stepped-squareshaped zones, maximum off-axis illumination is maintained up to themaximum angle allowed by the zone dimensions. Beyond these angles, thedegree of off-axis illumination is limited and can be tailored morespecifically for the x and y oriented geometry. FIGS. 9 and 10 shows howstepped-square zones are implemented in an illumination profile. FIG. 11shows how this is translated into the bi-level representation.

A square or rectangular shaped obscuration (or an inner limiting zone)emphasizes the off-axis illumination for feature pitch values whosefrequency distribution falls beyond the chosen value (greater thanlambda/(w*NA) where w is the fall width obscuration value between 0 and2). This is shown in FIG. 21 for a gaussian off axis distribution wherethe obscuration is 30% of the full aperture width. Combining a squareouter limiting zone and a square obscuration, an optimal condition ofoff axis illumination exists also. For features oriented in on directiononly, only two zones are needed on an axis opposite to the featuredirection. These zones can be slots or rectangles since spreading ofenergy in the direction of feature orientation is of no consequence toimaging performance and increases throughput. With two dimensionalgeometry, four slots are needed in x and y direction, resulting in asquare ring, as shown in FIG. 22. This ring can also be considered asthe combination of a square limiting zone and square obscuration. Thisrectangular ring source distribution can deliver off-axis illuminationfor features to 0.25 lambda/NA, depending on the choice of the limitingouter square zone. This square ring source distribution can also becombined with other off axis approaches, such as a gaussian four-zonedesign. FIG. 23 shows how a square ring source distribution is added toa gaussian four-zone design to produce results that are common to bothapproaches (that is better performance for more dense features out to0.25 lambda/NA and adequate through focus and through pitch imagingperformance.

Other combinations of source distributions are possible. FIG. 24 showshow an annular ring is combined with a gaussian four-zone design toemphasize the performance of more dense features than possible with theannular distribution alone. This approach can increase the off-axischaracter of the annulus and reduce the non-optimal on-axis character.

The masking aperture varies the intensity of the transmitted light atany element by modulating the state of pixels in each element. Thehighest intensity element has all pixels on or at maximum intensity.Light of suitable wavelength passes through without attenuation. In apreferred embodiment as shown in FIG. 12, an element with 64 pixels at aminimum intensity attenuates or blocks all light. Pixels of intensitybetween none and all are created by the number of pixels in a givenelement.

A masking aperture with a bilevel representation of such an illuminationdistribution is created by dithering the continuous tone images. Randomtechniques or fixed threshold techniques can be used. These fixedthreshold techniques are based on decision rules where any intensityvalue greater than a threshold value (T) results in a transparentmasking cell and a value less than T results in an opaque masking cell.The result is generally a high degree on banding in the bilevelrepresentation. A slight improvement over this method is to replace Twith equally distributed random numbers over the range 0 to 5 with a newrandom number generated for each intensity value. Less banding resultsbut signal to noise is low. A superior approach to the ditheredrepresentation is possible by comparing image intensity values toposition dependent thresholds contained in an n×n dither matrix, D″. Fora D″ matrix, a matrix element D″_(i,j) is chosen based on a rule setthat causes the dither matrix to be repeated in a checkerboard fashionover the entire image with minimum low spatial frequency noise. Theproper choice of the dither matrix results in minimum texture orartifacts and maximum uniformity in intensity. In general, the optimumdither matrix is represented by the recursion relationship:$D^{n} = {\begin{matrix}{{4D^{n/2}} + {D_{00}^{2}U^{n/2}}} & {{4D^{n/2}} + {D_{01}^{2}U^{n/2}}} \\{{4D^{n/2}} + {D_{10}^{2}U^{n/2}}} & {{4D^{n/2}} + {D_{11}^{2}U^{n/2}}}\end{matrix}}$where: $U^{n} = {\begin{matrix}1 & 1 & \cdots & 1 \\1 & \quad & \quad & \quad \\\vdots & \quad & \quad & \quad \\1 & \quad & \quad & \quad\end{matrix}}$To produce an 8×8 matrix to satisfy these optimization criteria, D8becomes: $D^{8} = {\begin{matrix}0 & 32 & 8 & 40 & 2 & 34 & 10 & 42 \\48 & 16 & 56 & 24 & 50 & 18 & 58 & 26 \\12 & 44 & 4 & 36 & 14 & 46 & 6 & 38 \\60 & 28 & 52 & 20 & 62 & 30 & 54 & 22 \\3 & 35 & 11 & 43 & 1 & 33 & 9 & 41 \\51 & 19 & 59 & 27 & 49 & 17 & 57 & 25 \\15 & 47 & 7 & 39 & 13 & 45 & 5 & 37 \\63 & 31 & 55 & 23 & 61 & 29 & 53 & 21\end{matrix}}$To utilize this dithering matrix, for example, a continuous tone elementintensity distribution is divided into 64 levels. The lowest intensitylevel places a pixel in the masking element at the zero position. Anintensity value of 50% places pixels in the first 31 positions, and soforth. FIG. 12 schematically depicts the bilevel representation of 65gray levels using the ordered dithering algorithm. FIG. 26 shows howthis approach is used in a 51×51 element array. Other possibilities,such as 2-level, 4-level, 16-level, and so forth, are solved for in asimilar manner.

The performance improvement for 130 nm features using a 193 nmwavelength and 0.60 lens numerical aperture is described using anormalized aerial image log-slope NILS metric (normalized to the featuresize). This is the log of the slope of the intensity image (or aerialimage). FIGS. 13 and 14 show NILS plotted against focus position forcircular zone and a bi-level representation of distributed-intensityzone off-axis illumination where the central intensity is 26%. Theratios are the line to space size ratio, or duty ratio. The falloff ofNILS across all feature duty ratios matches more closely with increasingamounts of defocus for the distributed-intensity zone illuminationcompared to the circular zone condition. The resulting impact onlithographic imaging is a reduction in the dense to isolated lineproximity effect. FIG. 15a shows the imaging improvement achieved withthe square limiting zone using the illumination profile of FIG. 19. FIG.15b shows the further improvement using the hybrid design of FIG. 23.FIGS. 15 a/d show a comparison of the square ring of FIG. 22 with anannular (circular) ring for a 248 nm wavelength, 0.63NA & 150 nmfeatures at 1:1.5 duty ratio to 1:5 duty ratio. FIG. 15c shows theperformance for an annular or circular ring and FIG. 15d shows theperformance for the square ring. Illuminator dimensions are identical,only the shape differs. No circular or annular ring can match theperformance of the square ring.

The amount of total intensity allowed to pass through a masking aperturewill determine its acceptability in situations where exposure throughputis a concern, such as with the fabrication of integrated circuitdevices. Table 2 is a comparison of the throughput efficiency of severalvariations of the distributed-intensity four-zone approach, measuredrelative to conventional illumination with a σ of 0.7 and a strongcircular zone approach, where off-axis illumination is provided by fourcircular zones on a zero transmission field. The worst case throughputis for the circular zone four-zone design, where the total intensitythrough the pupil is 27% of that for conventional illumination withσ=0.7. The distributed-intensity four-zone approach with a 0.7 σcircular hard stop leads to 83% throughput and the same design with a0.7 half-width square hard stop results in 85% throughput. If the squarelimiting zone is increased in size to 0.8 half-width, the throughputincreases to 93% and imaging performance remains comparable to thecircular hard stop. This efficiency comes about because of the amount ofenergy allowed at the corners of the square pupil, where the diagonalapproaches the full extent of the condenser lens pupil, or a σ valuenear 1.0. Comparison of intensity throughput is an important one asillumination modification is considered. If the illumination system ofan exposure tool can allow full value, σ=1 operation, this square hardstop variation of the distributed-intensity four-zone can lead tominimal losses.

TABLE 2 Relative Illumination approach Intensity Conventional, 0.7σ 1.00Circular four-zone 0.68σ_(c), 0.2σ_(r), 0.8σ stop, 0% field 0.27Circular four-zone, 0.68σ_(c), 0.2σ_(r), 0.8σ stop, 25% field 0.52Distributed-intensity four-zone, 0.68σ_(c), 0.3σ_(r), 0.7σ circular 0.83stop Distributed-intensity four-zone, 0.68σ_(c), 0.3σ_(r), 0.7σ square0.85 stop Distributed-intensity four-zone, 0.68σ_(c), 0.3σ_(r), 0.8σsquare 0.93 stop

For on-axis conventional illumination, lens aberrations are evaluatedassuming full use of a lens pupil. With off-axis illumination,diffraction information is distributed selectively over the lens pupil,influencing the impact of aberrations on imaging. In general, astigmaticeffects can worsen while spherical aberration and defocus effects can beimproved. Coma induced image placement can be further aggravated withOAI unless rebalanced with tilt. FIG. 14 shows how the normalized imagelog slope is impacted for circular four-zone (σ_(c)=0.7 and σ_(r)=0.2 ona zero intensity field) with 0.1 waves of primary aberrations;astigmatism, tilt, spherical, and coma, through focus for a 130 nmfeatures imaged with a 193 nm imaging system, using a numerical apertureof 0.6. Although the effects of spherical aberration are minimal, theloss in NILS for astigmatism, tilt, and coma are significant. FIG. 17shows the same imaging process using the bi-level representation of thedistributed-intensity four-zone illumination described in thisinvention. Similar results are obtained using dithered intensitycircular, stepped square, and 45 degree elliptical zones. Here it can beseen that the influences of astigmatism and tilt are reduced at zerodefocus values and at larger defocus values (both positive and negative)the influences of all aberrations are reduced substantially, as comparedto the case shown in FIG. 16.

Implementation of the invention into existing illumination systems isaccomplished via access to the illumination optical system. In oneembodiment, pixilated half-tone illumination files are transferredlithographically onto a transparent substrate, such as fused silica,coated with a suitably opaque masking layer, such as chromium. Aphotoresist film coated over the metal coated transparent substrate isexposed using optical, electron beam, or other methods by translatingthe bi-level illumination representation into a suitablemachine-readable format. Photoresist development and subsequent etchingof the underlying masking film allows transfer of the pattern to themasking aperture. An anti-reflective layer can be coated over themasking film prior to photoresist coating, exposure, and processing toreduce reflection, stray light, and flare effects in the illuminationfield of the exposure tool. An anti-reflective layer can be coated overthe patterned aperture to match reflectances over the entireillumination field. Alignment of apertures is made possible byincorporating alignment fiducials on the masking apertures and on anaperture holders used to mount the invention into the exposure toolillumination system. Apertures can be inserted into as pupil plane ofthe illumination system.

Those skilled in the art will understand that a number of the newresults achieved with the above-described aperture mask may also beachieved with a traditional beamsplitter illumination system. Forexample, consider an illumination system such as the one shown anddescribed in U.S. Pat. No. 5,627,625. When an aperture mask 60 with alarge, central square opening 64 and an opaque border 62 is insertedproximate to the fly's eye or in the lens pupil 19, the four beams fromthe beam splitter 16 generate contour and three dimensional plotssimilar to FIGS. 18 and 19. The beams fill the corners of theillumination pupil and limit the non-optimal frequency spreadingcharacter along the x and y axes while optimizing the off-axisillumination angles. The new results can also be achieved with anillumination system using diffractive optical elements (DOEs) to shapethe illumination profile. An example of this illumination shaping methodis described, for instance, in U.S. Pat. No. 5,926,257 (Canon) and U.S.Pat. No. 5,631,721 (SVGL). FIG. 25 shows how an aperture (60) with alarge, central square opening (64) and an opaque border (62) is insertedproximate to the beam shaping optical system (2). This can allow forshaping with a square limiting zone. Additionally, the beam shapingoptical system using diffractive optical elements can be tailored toproduce similar results. That is especially helpful for imaging featuresthat are oriented along x and y directions in the mask plane. The use ofa central obscuration (square and also round shaped) can similarly beachieved and can lead to performance improvements described here.Furthermore, any combination of off-axis illumination with a squareaperture or obscuration has potential to improve performance forgeometry oriented in the x-y direction. This can include, but is notlimited to, round zones, elliptical zones, square zones, and annularslots (that is an annular ring masked off on x and y axis to formarc-shaped zones).

The masking aperture described above may be used alone or in combinationwith prior art techniques and apparatus. For example, the maskingaperture of the invention may be combined with the four hole metal maskdescribed in JP patent Laid-Open (KOKAI) Publication No. 4-267515 anddiscussed in the background of U.S. Pat. No. 5,627,625.

Those skilled in the art will understand that the new results achievedwith elliptical, 45 degree elliptical, square rings, and square shapedzones may also be achieved with alternative approaches, including thebeamsplitter approach. This could be accomplished for example with theillumination system shown in FIG. 20 (and described in U.S. Pat. No.5,627,625) by shaping divided beams into elliptical, 45 degreeelliptical, and square shapes in the beamsplitter unit 16 andsuperposing them using a prism unit 17. Gaussian or similar shapedenergy distribution is possible.

Those skilled in the art will also understand that the new resultsachieved with elliptical, 45 degree elliptical, square rings, and squareshaped zone may also be achieved with diffractive optical elementapproaches to beam shaping, such as that described in U.S. Pat. Nos.5,926,257 and 5,631,721 by tailoring the diffractive optical elements toexhibit these characteristics. The micro-diffractive optical elementswithin the beam-shaping optical system (2) in FIG. 25 are manipulated toallow for the required shaping. It is well known that diffractiveoptical elements (DOEs) and halographic optical elements (HOEs) canallow for efficient manipulation of arbitrary wavefronts with moreflexibility and reduced fabrication requirements compared toconventional refractive optics (see for instance Z. Yang and K.Rosenbruch, SPIE Vol. 1354, (1990), 323). One DOE or HOE or thecombination of two or more DOEs or HOEs allow for the design flexibilityneeded to achieve the desired results, as demonstrated for instance inU.S. Pat. No. 5,926,257 and in SPIE Vol. 1354, (1990), 323.

The present invention is described above but it is to be understood thatit is not limited to these descriptive examples. The numerical values,number of zones, shapes, and limiting zones may be changed toaccommodate specific conditions of masking, aberration, featureorientation, duty ratio requirements, lens parameters, initialillumination non-uniformities, and the like as required to achieve highintegrated circuit pattern resolution. Results can also be obtained bycontrolling illumination at any Fourier Transform plan in theillumination system.

1. A method for controlling on-axis and off-axis illumination of aphotomask comprising the steps of : directing a beam of light of aselected wavelength toward a pupil of an illumination system; passingthe beam of light through a fly's eye lens located near the pupil;diffracting the light through a masking aperture having a half tonediffraction pattern of dithered pixels patterned for distributing todistribute the light into two or more zones.
 2. The method of claim 1wherein said half-tone diffraction pattern of dithered pixels comprisesan array of pixels, each pixel of a clear or opaque type, said clear andopaque pixels for respectively passing and blocking incident light,wherein the number, size, and type of the pixels are chosen inaccordance with: (a) the wavelength of light used to illuminate thephotomask, and (b) the a size and shape of the features of thephotomask, for generating to generate a continuous illuminationintensity pattern on the photomask with illumination intensity at anylocation controlled by the half-tone dithered image .
 3. The maskingaperture method of claim 2 wherein the half-tone dithered image maskingaperture comprises an array of diffraction elements and each diffractionelement is comprises a dithered image pattern of clear and/or opaquepixels.
 4. The method of claim 3 wherein each diffraction elementcomprise an n×n dithered matrix of pixels, the an intensity of eachelement is defined by the number and type of pixels in its ditheredmatrix and wherein the pixels in each matrix are dithered to avoidartifacts.
 5. The method of claim 4 wherein the matrix of diffractingelements pixels is selected from the group consisting of 2×2, 4×4, 8×8,16×16, 32×32 and 64×64 pixels.
 6. The method of claim 3 wherein the anintensity of each sub pixel is defined by a recursion relationshipwhere: ${\begin{matrix}{{4D^{n/2}} + {D_{00}^{2}U^{n/2}}} & {{4D^{n/2}} + {D_{01}^{2}U^{n/2}}} \\{{4D^{n/2}} + {D_{10}^{2}U^{n/2}}} & {{4D^{n/2}} + {D_{11}^{2}U^{n/2}}}\end{matrix}}\quad$ where $U^{n} = {{\begin{matrix}1 & 1 & \cdots & 1 \\1 & \quad & \quad & \quad \\\vdots & \quad & \quad & \quad \\1 & \quad & \quad & \quad\end{matrix}}.}$
 7. The method of claim 6 wherein the matrix of pixelscomprises an 8×8 matrix and the relative intensity, D8, comprises:$D^{8} = {{\begin{matrix}0 & 32 & 8 & 40 & 2 & 34 & 10 & 42 \\48 & 16 & 56 & 24 & 50 & 18 & 58 & 26 \\12 & 44 & 4 & 36 & 14 & 46 & 6 & 38 \\60 & 28 & 52 & 20 & 62 & 30 & 54 & 22 \\3 & 35 & 11 & 43 & 1 & 33 & 9 & 41 \\51 & 19 & 59 & 27 & 49 & 17 & 57 & 27 \\15 & 47 & 7 & 39 & 13 & 45 & 5 & 37 \\63 & 31 & 55 & 23 & 61 & 29 & 53 & 21\end{matrix}}.}$
 8. The method of claim 1 wherein the zones arearranged symmetrically about the center of the masking aperture.
 9. Themethod of claim 1 wherein the zones are arranged asymmetrically aboutthe center of the masking aperture.
 10. The method of claim 1 whereineach of the zones have has one shape selected from the group consistingof circles , squares , rectangles , ellipses , rings , circular rings ,square rings and any combinations thereof.
 11. The method of claim 1 10wherein the selected shape is a stepped square.
 12. The method of claim1 wherein the zone(s) each zone is shaped in an ellipse and the majoraxis of each ellipse is aligned at a 45 degree angle with respect to thecenter of the masking aperture.
 13. A method for controlling on-axis andoff-axis illumination of a photomask comprising: directing a beam oflight of a selected wavelength toward a pupil of an illumination system;passing the beam of light through a fly's eye lens located near thepupil; diffracting the light through a masking aperture having a halftone diffraction pattern of dithered pixels patterned for distributingto distribute the light into one or more zones to form a pattern ofillumination intensity in the one or more zones wherein each zone has ashape corresponding to shapes selected from the group consisting ofellipses , square rings , stepped squares and any combination thereof.14. The method of claim 13 further comprising diffracting the light beamthrough a masking aperture having a half tone diffraction pattern ofdithered pixels patterned for distributing to distribute the light intoone or more zones to form one or more additional patterns of lightintensity selected from the group consisting of circles , squares ,rectangles and circular rings .
 15. A method for controlling on-axis andoff-axis illumination of a photomask comprising: providing a beam oflight; homogenizing the light with an optical element; and diffractingthe light with a half tone diffraction pattern of dithered pixelspatterned to distribute the light into two or more zones.
 16. The methodof claim 15 wherein said half-tone diffraction pattern of ditheredpixels comprises an array of pixels, each pixel of a clear or opaquetype, said clear and opaque pixels for respectively passing and blockingincident light, wherein the number, size, and type of the pixels arechosen in accordance with: (a) a wavelength of light used to illuminatethe photomask, and (b) a size and shape of features of the photomask, togenerate a continuous illumination intensity pattern on the photomask.17. The method of claim 16 wherein a masking aperture used in thediffracting comprises an array of diffraction elements and eachdiffraction element comprises a dithered pattern of clear and/or opaquepixels.
 18. The method of claim 17 wherein each diffraction elementcomprises an n×n dithered matrix of pixels, an intensity of each elementis defined by the number and type of pixels in its dithered matrix andwherein the pixels in each matrix are dithered to avoid artifacts. 19.The method of claim 18 wherein the matrix of pixels is selected from thegroup consisting of 2×2, 4×4, 8×8, 16×16, 32×32 and 64×64 pixels. 20.The method of claim 17 wherein an intensity of each pixel is defined bya recursion relationship where: $D^{n} = {\begin{matrix}{{4D^{n/2}} + {D_{00}^{2}U^{n/2}}} & {{4D^{n/2}} + {D_{01}^{2}U^{n/2}}} \\{{4D^{n/2}} + {D_{10}^{2}U^{n/2}}} & {{4D^{n/2}} + {D_{11}^{2}U^{n/2}}}\end{matrix}}$ where: $U^{n} = {{\begin{matrix}1 & 1 & \cdots & 1 \\1 & \quad & \quad & \quad \\\vdots & \quad & \quad & \quad \\1 & \quad & \quad & \quad\end{matrix}}.}$
 21. The method of claim 20 wherein the matrix ofpixels comprises an 8×8 matrix and the relative intensity, D⁸ ,comprises: $D^{8} = {{\begin{matrix}0 & 32 & 8 & 40 & 2 & 34 & 10 & 42 \\48 & 16 & 56 & 24 & 50 & 18 & 58 & 26 \\12 & 44 & 4 & 36 & 14 & 46 & 6 & 38 \\60 & 28 & 52 & 20 & 62 & 30 & 54 & 22 \\3 & 35 & 11 & 43 & 1 & 33 & 9 & 41 \\51 & 19 & 59 & 27 & 49 & 17 & 57 & 25 \\15 & 47 & 7 & 39 & 13 & 45 & 5 & 37 \\63 & 31 & 55 & 23 & 61 & 29 & 53 & 21\end{matrix}}.}$
 22. The method of claim 15 wherein the zones arearranged symmetrically about the center of a masking aperture used inthe diffracting.
 23. The method of claim 15 wherein the zones arearranged symmetrically about the center of a masking aperture used inthe diffracting.
 24. The method of claim 15 wherein each of the zoneshas a shape selected from the group consisting of circle, square,rectangle, ellipse, ring, circular ring, square ring and any combinationthereof.
 25. The method of claim 24 wherein the selected shape is astepped square.
 26. The method of claim 15 wherein each zone is shapedin an ellipse and the major axis of each ellipse is aligned at a 45degree angle with respect to the center of a masking aperture used inthe diffracting.
 27. A method for controlling on-axis and off-axisillumination of a photomask comprising: providing a beam of light;homogenizing the light with an optical element; and diffracting thelight with a half tone diffraction pattern of dithered pixels patternedto distribute the light into one or more zones to form a pattern ofillumination intensity in the one or more zones wherein each zone has ashape selected from the group consisting of ellipse, square ring,stepped square and any combination thereof.
 28. The method of claim 27further comprising diffracting the light beam with a half tonediffraction pattern of dithered pixels patterned to distribute the lightinto one or more zones to form one or more additional patterns of lightintensity selected from the group consisting of circle, square,rectangle and circular ring.